ds2782 Maxim Integrated Products, Inc., ds2782 Datasheet - Page 13

• Manufacturer Part Number: ds2782
• Description: Ds2782 Stand-alone Fuel Gauge Ic
• Manufacturer: Maxim Integrated Products, Inc.

MODELING CELL CHARACTERISTICS
In order to achieve reasonable accuracy in estimating remaining capacity, the cell performance characteristics over
temperature, load current, and charge termination point must be considered. Since the behavior of Li-ion cells is
non-linear, even over a limited temperature range of 10°C to 35°C, these characteristics must be included in the
capacity estimation to achieve a reasonable accuracy. See Applications Note AN131 “Li+ Fuel Gauging with Dallas
Semiconductor Devices” for general information on the FuelPack™ method used in the DS2782. To facilitate
efficient implementation in hardware, a modified version of the method outlined in AN131 is used to store cell
characteristics in the DS2782. Full and empty points are retrieved in a lookup process which re-traces a piece-wise
linear model. Three model curves are stored: Full, Active Empty and Standby Empty. Each model curve is
constructed with 4 line segments and spans from 0°C to 40°C. Operation outside the 0°C to 40°C model span is
supported by the model with minimal loss of accuracy. Above 40°C, the 40°C fixed points are extended with zero
slope. This achieves a conservative capacity estimate for temperatures above 40°C. Below 0°C, the model curves
are extended using the slope of each 0°C to 10°C segment. If low temperature operation is expected, the 0°C to
10°C slopes can be selected to optimize the model accuracy. A diagram of example battery cell model curves is
shown if Figure 12.
Figure 12. Cell Model Example Diagram
Full: The Full curve defines how the full point of a given cell depends on temperature for a given charge
termination. The charge termination method used in the application is used to determine the table values. The
DS2782 reconstructs the Full line from cell characteristic table values to determine the Full capacity of the battery
at each temperature. Reconstruction occurs in one-degree temperature increments.
Active Empty: The Active Empty curve defines the temperature variation in the empty point of the discharge profile
based on a high level load current (one that is sustained during a high power operating mode) and the minimum
voltage required for system operation. This load current is programmed as the Active Empty current (IAE), and
should be a 3.5s average value to correspond to values read from the Current register, and the specified minimum
voltage, or Active Empty voltage (VAE) should be a 220ms average to correspond to values read from the Voltage
register. The DS2782 reconstructs the Active Empty line from cell characteristic table values to determine the
Active Empty capacity of the battery at each temperature. Reconstruction occurs in one-degree temperature
increments.
Standby Empty: The Standby Empty curve defines the temperature variation in the empty point in the discharge
defined by the application standby current and the minimum voltage required for standby operation. In typical PDA
FuelPack is a trademark of Dallas Semiconductor.
100%
0°C
1
Derivative
[ppm / °C]
10°C
2
20°C
3
13 of 25
Standby
Empty
Active
Empty
FULL
30°C
4
40°C
Cell
Characterization
data points
DS2782: Standalone Fuel Gauge IC